PHP 7.3 is alread in RC stage, so it seems to be reasonable to check CMSimple_XH and plugins/addons/templates for potential incompatibilities. The notable changes are listed in the UPGRADING document. Most relevant for us may be:

Probably not much to worry about, but item #6 might be something that we'd like to implement.

The biggest issue I currently see, is that uopz is still not compatible with PHP 7.3, so that the CMSimple_XH unit tests can't be run against PHP 7.3. Also this makes it hard for me to release a Portable_XH version with PHP 7.3.